She did the first time someone got on and sometimes she has sudden explosions I havent tried it but do you think maybe we should try desensitizing her???? Do you think it would help with her getting spooked as much
I think you need to start from the ground up on this horse...if she is so terrified and explosive undersaddle, then she has obviously missed steps getting there in the first place.
I would concentrate on ground work only, desensitizing her to everything and anything, for a good couple of weeks. This will also give her mind a 'break' from the explosions she's experienced. Focus on building a trusting working relationship on her, then start retraining her to saddle and rider, and treat her as if she's never had anyone on her before; go in tiny steps...get her used to the mounting block, step in the stirrup oodles of times just to get her used to the sensation of someone stepping up, then start laying across her back, etc...
This horse sounds like she's just plain confused by what's going on, so taking her back through everything is going to be essential to making her a calm and relaxed partner.